The preparation of polyether carbonate polyols by catalytic reaction of alkylene oxides (epoxides) and carbon dioxide in the presence of H-functional starter substances (“starters”) has been the subject of intensive study for more than 40 years (e.g. Inoue et al., Copolymerization of Carbon Dioxide and Epoxide with Organometallic Compounds; Die Makromolekulare Chemie 130, 210-220, 1969). This reaction is shown in schematic form in scheme (I), where R is an organic radical such as alkyl, alkylaryl or aryl, each of which may also contain heteroatoms, for example O, S, Si etc., and where e, f and g are integers, and where the product shown here in scheme (I) for the polyether carbonate polyol should merely be understood such that blocks having the structure shown may in principle be present in the polyether carbonate polyol obtained, but the sequence, number and length of the blocks and OH functionality of the starter can vary, and is not limited to the polyether carbonate polyol shown in scheme (I). This reaction (see scheme (I)) is environmentally very advantageous since this reaction constitutes the conversion of a greenhouse gas such as CO2 to a polymer. A further product, actually a by-product, formed is the cyclic carbonate shown in scheme (I) (for example, when R═CH3, propylene carbonate).

EP-A 0 222 453 discloses a process for preparing polycarbonates from alkylene oxides and carbon dioxide using a catalyst system composed of DMC catalyst and a cocatalyst such as zinc sulfate. The polymerization is initiated here by once contacting a portion of the alkylene oxide with the catalyst system. Only thereafter are the remaining amount of alkylene oxide and the carbon dioxide metered in simultaneously. The amount of 60% by weight of alkylene oxide compound relative to the H-functional starter compound, as specified in EP-A 0 222 453 for the activation step in examples 1 to 7, is high and has the disadvantage that this constitutes a certain safety risk for industrial scale applications because of the high exothermicity of the homopolymerization of alkylene oxide compounds.
WO-A 2008/092767 discloses a process for preparing polyether carbonate polyols, characterized in that one or more H-functional starter substances are initially charged in the reactor and that one or more H-functional starter substances are metered continuously into the reactor during the reaction. In the examples, WO-A 2008/092767 discloses a process (in the context of the present invention a CAOS semi-batchwise process) in which H-functional starter substance and DMC catalyst are initially charged and, under a CO2 atmosphere, propylene oxide and H-functional starter substance are metered in intermittently, wherein the addition of propylene oxide and H-functional starter substance is started at a free propylene oxide content of less than 3% by weight, and wherein the addition of these substances is stopped at a free propylene oxide content of 8% by weight.